Viable book, absolutely, but it has to be viable in terms of the market. At a time when, in the last decade, new SF writers have included Alastair Reynolds, Richard Morgan, Justina Robson, Charles Stross, Liz Williams, Jon Courtenay Grimwood and Neal Asher - and Fantasy has turned up China Mieville, Scott Lynch, George R R Martin, Joe Abercrombie and others, I don't feel anyone can say the market is constricted.
But every editor turns down books they love personally, because their head knows they aren't commercial. The writing is great, but the basic story isn't strong enough, for instance, and the book would need to be rewritten word by word and lengthened by 30,000 words. At that point you make it clear to the author and their agent that you want to see more from the author and you'd be happy to discuss future projects face-to-face, but not this book.
The editor's job, as far as their MD is concerned, is to sniff out bestsellers - commercial publishing is about the bottom line.